Method of sealing a permeable prous medium



United States Patent 3,302,734 METHOD OF SEALING A PERMEABLE POROUSMEDIUM Victor G. Meadors, Tulsa, Okla., assignor, by mesne assignments,to Esso Production Research Company, a

corporation of Delaware No Drawing. Filed July 1, 1963, Ser. No. 292,108

8 Claims. (Cl. 175-59) The present invention is generally concerned withthe sampling of underground formations. The invention particularlyrelates to a method for obtaining samples of subterranean strata fromwellbores which have been drilled into the earths crust. It isespecially directed to a method for obtaining samples of essentiallyunaltered fluid content by a rotary core drilling technique whichinvolves the use ofa non-invading coring fluid to seal the surfaces ofthe core samples essentially as drilled.

The stock-tank volume of petroleum contained in a porous undergroundreservoir is normally calculated in terms of the reservoir volume, theformation volume factor, the average reservoir porosity and the averageoil saturation within the pore volume. In reservoirs wherein a number ofdevelopment wells have been drilled, the reservoir volume and theformation volume factor can usually be determined with reasonableaccuracy by comparing structural maps and by measuring the gravity,temperature, pressure and gas content of the oil under reservoirconditions. The average porosity of the reservoir is generallydetermined by analyzing cores recovered from a number of developmentwells. Experience has demonstrated, however, that volumetric oil contentin the cores as determined from core analysis is usually not an accurateindication of the actual quantity of oil present in a reservoir.

Reliable values for volumetric oil content, together with the otherdata, are extremely desirable to forecast the productive life of oil andgas reservoirs, to select the primary recovery techniques most suitablefor particular reservoirs, and to assess the susceptibility of suchreservoirs to later secondary and tertiary recovery processes. It is aprincipal object of the invention to obtain samples from a reservoir,the analysis of which will provide reliable values for volumetric oilcontent.

Conventional core drilling systems include an annular bit and corebarrel which are rotated from the earths surface by means of a rotarydrill string. A coring fluid is circulated downwardly through passagesin the drill string, barrel, and bit in order to maintain pressure onthe formation and thus prevent the escape of fluids contained therein.Cuttings produced by the bit are entrained in the coring fluid andreturned to the surface through the annulus surrounding the drillstring. As the bit cuts away the formation the central core whichremains is encased in the barrel. The core barrel is provided with meansfor breaking off the core once the barrel has been filled. Pressure corebarrels which can be sealed against changes in pressure are also used.After the core has been cut the drill string is withdrawn from theborehole and the core recovered therefrom.

Studies have shown that the pressure maintained at the bottom of aborehole during a coring operation has a profound effect on the fluidcontent of the cores subse- "ice quently recovered. If this pressure: isless than the formation pressure,.fluids contained in the formation willtend to flow out of the core into the borehole until equilibrium isestablished. If on the other hand the bottom hole pressure exceeds theformation pressure, the coring fluid will tend to flow into theinterstices of the formation and displace any oil, gas or watercontained therein. In either case the result is a change in the fluidcontent of the core such that subsequent measurements of the amount offluids present will not accurately reflect the original fluid content ofthe cored formation. Since this change in fluid content occurscontinuously as the core is cut, the use of a pressure core barrel doesnot prevent it.

Several methods for avoiding the difficulty outlined above have beenproposed in the past. The most obvious of these involves carrying outcoring operations without any pressure differential between the coringfluid and the formation. This is impractical, if not impossible, becausethe formation pressure cannot be conveniently measured during coredrilling, and moreover, because the coring fluid pressure cannot becontrolled with suflicient accuracy.

The use of coring fluids which will not invade the formation underpressures well in excess of the formation pressure has been suggested,but efforts to develop a satisfactory non-invading fluid have not beenentirely successful. Mercury, molten metal and other materials advocatedin the past all invade the formation to an appreciable extent and hencecause changes in the fluid content. In addition the materials proposedfor this purpose have generally been costly and diificult to use and inmany cases require highly specialized core bits and core barrels. Othersystems including the use of tracers which permit determination of theextent to which core invasion has occurred, and systems for freezing thecore have been proposed but have not been found to be generallyeflective.

The most promising of the non-invading coring fluids which have beendeveloped heretofore are the polymeric elastomer latices. Thesematerials are aqueous colloidal dispersions. of polymeric elastomers,including natural and synthetic rubber latices.

It has now been found that aqueous dispersions or suspensions of finelydivided reclaimed rubber are unexpectedly superior to the natural andsynthetic rubber latices, in their ability to seal a permeable porousmedium. Specifically, a field-scale rotary coring operation in which areclaimed rubber dispersion is used as the coring fluid has been foundcapable of recovering a core sample in which the original fluids contentis altered by less than 3% by volume, even when the operation isconducted with as much as 500 psi. pressure differential between thecoring fluid and the fluids of the formation being cored.

- Reclaimed rubber is a staple item of commerce, produced by any one ofvarious processes, each of which involves the application of heat andchemical agents to Waste vulcanized rubber, whereby a substantialdevulcanization or-regeneration of the rubber to a plastic state isachieved. It may then be processed, compounded, and again vulcanized,similarly as is true of new rubber.

The reclaiming process, from a chemical viewpoint, is

not an exact reversal of the reactions which occur during thevulcanization of natural rubber or synthetic rubber; hence, the termdevulcanization has been criticized as misleading and inaccurate.However, the term persists in the literature and is considered to be avalid description of the reclaiming process, inasmuch as a reversal ofphysical properties does occur. That is, vulcanized rubber ischaracterized by elasticity and a resistance to compression, stretchingand swelling, whereas the devulcanized product is restored to a state ofplasticity, more nearly like that of unvulcanized new rubber.

Nevertheless, reclaimed rubber has many unique properties whichdistinguish it from new rubber. For example, in the processing of arubber compound, the presence of reclaim in the recipe enhances workingproperties in all primary stages. Mixing times have been halved andcalender speeds doubled. Calender temperatures become less critical, andthe gauge of a calendered sheet can be increased without blistering. Theuncured stock possesses better dimensional stability and is lesssusceptible to overmilling. Friction and skim-coating operations areexpedited.

Reclaim stocks possess a high rate of cure. They often exhibit improvedmoulding properties. They possess low thermoplasticity and are lessaffected by continuous millmg.

There are many types and grades of reclaimed rubber. The most abundantsource of reclaimed rubber is old tires, including passenger car andtruck tires. Reclaim types produced from tires include the whole tirereclaim, the tire tread reclaim, and the tire carcass reclaim. There isalso a red inner tube reclaim and a black inner tube reclaim.

Other types include the mechanical reclaim, the footwear reclaim, thebutyl reclaim, the neoprene reclaim and the nitrile reclaim, as well asseveral others.

In its broadest scope, the present invention includes the use of aqueousdispersions of any of the above types of reclaimed rubber. It alsoincludes the use of mixtures of two or more reclaim types, and mixturesof one or more reclaim types with one or more of the natural andsynthetic elastomer latices. In the latter case, however, it ispreferred that the mixture contain a major proportion of reclaimedrubber.

Suitable reclaimed rubber dispersions for use in accordance with theinvention are readily .available commercially. For example, there is theLoxi-te series of the Firestone Tire and Rubber Co., and the Dispersiteseries of the Us. Rubber Co. Reclaim dispersions are prepared by firstmechanically milling or grinding the reclaimed rubber until itsconsistency becomes suitably plastic for dispersing water therein. Anemulsifying agent is then blended into the rubber, after which water isslowly added. Once the correct amount of water has been added, a changeof phase takes place, whereby the water becomes the continuous phase,and the rubber becomes the discontinuous, or dispersed phase.

Dispersions containing from about 20% to about 75% reclaim by weight aregenerally suitable for purposes of the invention. Those containing fromabout 35% to about 70% are preferred.

Reclaim rubber dispersions are chanacterized by a relatively widedistribution of particle sizes, ranging from as small as 0.1 micron indiameter to as large as 35 microns. The average particle size ofcommercially available reclaim dispersions is subject, however, torelatively little variation since as a practical matter, the particlesize is determined by such factors as the amount and type of emulsifierused, and the point at which inversion from a water-in-oil type to anoil-in-water type of dispersion occurs.

Core drilling operations utilizing the coring fluid of the invention maybe carried out with conventional apparatus familiar to those skilled inthe art. A variety of commercially available core bits and core barrelsmay be used. The coring fluid is circulated down the drill stringthrough channels in the core barrel and bit so that it emerges adjacentthe cutting surfaces of the bit. The fluid contacts the core surfaces asthe surrounding rock is cut away and continuously forms a film on thesurface which is impermeable to oil, gas and water, thus preventing anysignificant alteration of the original fluid saturations in the core.Once a coated core of suflicient length has been cut, it is separatedfrom the formation, and lifted to the surface inside the core barrel.

The nature and objects of the invention can be more fully understood byreference to the following experimental work, conducted to demonstratethe effectiveness of the invention.

Static invasion tests were conducted to compare the sea-ling ability ofa reclaimed rubber dispersion with the corresponding eflectiveness ofnatural and synthetic rubber latices. Berea sandstone cores of about 1.5inches in diameter and about one inch in length, having a porosity ofabout 20% and a permeability ranging from 200 to 300 millidarcies, wereused in testing the fluids. Each core was first saturated with a mixtureof oil and brine, to simulate field conditions, and then mounted in aconventional core holder. As summarized in Table I, the cores were thenexposed to the test fluids, under a driving force of p.s.i.,differentialpressure.

TABLE I Filtrate invasion rate, milliliters per minute Coring Fluid:after five minutes exposure 2 parts Nitrex 2620, 1 part natural latex,4.4%

silica flour 0.02

2 parts Naugatex 2001, 1 part natural latex,

4.4% silica flour 0.02 50/50 (vol.) Dispersite 1685-natural latex 0.01Dispersite 1685 0.01

Dispersite 1685 is a trademark of Naugatuck Chemical Company, a divisionof the US. Rubber Co., Naugatuck, Connecticut. The product is arubber-in-water dispersion prepared from first quality whole tirereclaim, and having a total solids content of about 60 weight percent.

Nitrex 2620 is a latex prepared by the emulsion polymerization of 65weight percent butadiene and 35 Weight percent acrylonitrile in thepresence of a persulfate catalyst and about 8 weight percent ofpotassium rosin soap emulsifier. It has a 40 weight percent solidscontent and an average particle diameter of 0.07 microns.

Naugatex 2001 is an SBR latex the elastomer of which contains 46 percentbound styrene. The latex has a total solids content of 42 percent byweight, and a particle size of about 0.10 micron.

In another series of tests, cores were drilled from blocks of poroussandstone using conventional rotary core-drilling equipment. Thesandstone blocks measured 12 inches square by 24 inches deep, and wereprepared for the tests by replacing the natural fluids contained thereinwith carefully measured amounts of oil and brine. A typical blockcontained 60 percent oil and 40 percent brine. The oil employed was a 12centipoise white paraflinic hydrocarbon oil. The brine contained 2,500parts per million of calcium ion, 1,000 parts per million magnesium ion,and 26,000 parts per million sodium ion, present as the chloride salts,which is typical of oil fie-ld brines.

The sandstone block, the core bit and the core barrel were encased in apressure-tight chamber to permit the simulation of high formationpressures. The coring fluids were circulated from a reservoir throughthe drill pipe, core barrel, and core bit by a high pressure pump. Thecuttings were lifted by the fluids through the annulus surrounding thedrill string, after which the fluids were freed of cuttings andrecirculated. The coring fluids were circulated at a pressuredifferential of about 75 p.s.i. in excess of that maintained within thesandstone block. The circulation rate was about 38 gallons per minute.This particular equipment and these conditions were selected becauseearlier tests had shown that the results obtained were comparable tothose obtained in actual field operations. The results are summarized inthe following table.

TABLE II Goring Fluid 50/50 (vol) 50/50 (vol.) 2 Parts Nitrex DispersiteDispersite 26:20, 1 Part 1G85Natural 1685Natural Natural Rubber LatexRubber Latex Rubber Latex Total Solids, percent- 64. 8 64. 8 51 Bit,r.p.m 30 30 30 Bit Weight lbs 10, 000 10,000 10,000 Inches eore(l. l5. 716. 6 16.8 Goring Time, min 6. 5 5. 3 5. 6 S0 in Rock, percent PV 59. 445. 2 62. 2 So in Core, percent PV 52.0 35. 4 46. 4 Difference, percentThe data summarized in Table II further demonstrate the superiority ofreclaimed rubber dispersions over the natural and synthetic rubberlatices.

Additional tests were conducted, in which a core sample, or series ofcore samples, was drilled with conventional field-scale rotarycore-drilling equipment from an octagonal block of Berea sandstonemeasuring four feet in length and ten inches between parallel sides.Each block was first cemented inside a length of 11% inch diameter steelcasing which was equipped with suitable fittings to :permit thesimulation of high formation pressures while drilling. The results aresummarized in Table III.

TABLE III [Goring fluidDispersite D-735] Dispersite D-735 is an aqueousreclaimed rubber dispersion containing -65% by weight of oil-extendedfirst quality whole tire reclaim.

These data further indicate the superiority of reclaimed rubberdispersions as coring fluids in the recovery of core samples ofsubstantially unaltered fluids content.

What is claimed is:

1. A process for the recovery of core samples from a borehole in theearth which comprises advancing a rotary core-bit into the earth at thebottom of said borehole, while circulating downhole a fluid comprising adispersion of finely divided reclaimed rubber in water, whereby thesample recovered are sealed substantitally as cut, thereby capturing theoriginal fluids contained therein.

2. A process as defined by claim 1 wherein the dispersed phase of saiddispersion comprises first quality whole tire reclaim.

3. A process as defined by claim 1 wherein the rubber content of saiddispersion is 35 to by weight.

4. A process as defined by claim 1 wherein the dispersed phase of saiddispersion comprises reclaim rubber plus natural latex, and the combinedrubber content thereof is 35 to 70% by Weight.

5. In the recovery of core samples from a borehole in the earth byrotary-core-drilling wherein a sealing fluid is circulated downhole, theimprovement which comprises circulating as said fluid a compositioncomprising a dispersion of finely divided reclaimed rubber in water.

6. A process as defined by claim 5 wherein said dispersion containsfirst quality whole tire reclaim.

7. A process as defined by claim 5 wherein the dispersed phase of saidcomposition consists essentially of first quality whole tire reclaim.

8. A process as defined by claim 5 wherein the rubber content of saiddispersion is in the range of 35% to 70% by weight.

References Cited by the Examiner UNITED STATES PATENTS 3/1964 Ga'llus 5911/1964 Gallus 175-59

1. A PROCESS FOR THE RECOVERY OF CORE SAMPLES FROM A BOREHOLE IN THEEARTH WHICH COMPRISES ADVANCING A ROTARY CORE-BIT INTO THE EARTH AT THEBOTTOM OF SAID BOREHOLE, WHILE CIRCULATING DOWNHOLE A FLUID COMPRISING ADISPERSION OF FINELY DIVIDED RECLAIMED RUBBER IN WATER, WHEREBY THESAMPLE RECOVERED ARE SEALED SUBSTANTIALLY AS CUT, THEREBY CAPTURING THEORIGINAL FLUIDS CONTAINED THEREIN.